Class IndexSorter

Sort an index by document importance factor. Higher scoring documents are
assigned smaller document numbers. Document weights are obtained from a
specified field, which has to be single-valued and stored, with string value
that represents a float number. Stored fields in the output index remain
consistent, i.e. both stored fields and postings are renumbered in sync.

NOTE: this tool is unaware of documents added
atomically via IndexWriter.addDocuments(java.util.Collection<org.apache.lucene.document.Document>) or IndexWriter.updateDocuments(org.apache.lucene.index.Term, java.util.Collection<org.apache.lucene.document.Document>), which means it can easily
break up such document groups.